In optical communications and data transmission systems it is frequently required to couple light energy out of an optical path carrying signal information and also to controllably and selectively modulate the light energy propagating along such an optical path. In the prior art, light energy has been coupled out of an optical path in such systems through the use of a grating coupler, employing a periodic diffraction grating on the surface of the material which comprises the optical path, such as the core area of a fiber optic cable having the cladding removed.
Another prior art alternative technique employed to couple light out of an optical path is the use of a prism coupler. The prism coupler technique is practiced by situating a prism at a certain critical distance proximate to the optical path so that frustrated photo reflection is upheld. In a practice of this latter technique the magnitude of light leakage is dependent upon the thickness and refractive index of a material which comprises the optical path, the refractive index of the prism and the waveguide, the angle of the incident beam, and the wavelength of the light energy involved, as well as its plane of polarization. The critical dependence upon these several parameters in the employment of the prism type optical coupler gives rise to a number of disadvantages; these include (1) critical air gap tolerance (2) only one state of operation unless very sophisticated mechanical placement devices are used to move the prism in and out relative to the optical path, and (3) data rate limitation for an active device.
Inherent in all such devices also is the very important and basic problem of an extremely high degree of criticality of dimensional tolerances involving measurements of a microscopic order or less. This basic problem inheres particularly in the fabrication, use, and operation of optical waveguides fabricated by conventional diffusion and deposition techniques, for example.
Previously described techniques for fabrication of optical waveguides having a pre-established specified coupling coefficient between them relied upon extremely precise control of the spatial disposition in the form of separation between the waveguides. Moreover, the coupling coefficient is an extremely sensitive function of waveguide separation for the close proximities required to achieve satisfactory coupling. The very stringent dimensional tolerances required for the fabrication of such optical couplers cannot be achieved satisfactorily with a simple photolithographical fabrication technique, for example.
Additionally, coupling between two co-extensive waveguides could only be changed in prior art practices in different parts of the same device by varying the waveguide separation. Thus, the regions of changing separation are in effect transition portions between no coupling and coupling with the result that the total coupling between the waveguides becomes less well defined for design purposes because of the presence of such transition regions.
Accordingly, there is a need for an optical coupler between two optical paths comprising optical waveguides in which the degree of coupling is readily controllable to a high degree of precision through the use of fabrication methods and techniques which avail of significantly less stringent dimensional tolerances than the methods and techniques known and used in functionally comparable prior art devices.
For instance, the concept and teaching of the present invention avoids the requirement for strict tolerances involving extremely small separations between co-linear, co-extensive optical waveguides in the prior art which required the use of scanning electron microscope techniques to achieve optimum designs and satisfactory performance.